Derivation of the potential of a sphere

I've been told in a lecture course that the potential used in the virial theorem (for our application) is 3/5 GM/r. This describes the potential for a sphere of radius r, mass M created by bringing infinitly thin shells from infinity to form the next 'layer' of the sphere. I am having difficulty deriving this for myself, anybody wana give it a try ??

The first one is just correcting the result. I'm pretty sure he had mis-typed it. The second is summing over the potentials of spherical shells at a radius r and with a width of dr (brought in from infinity and assuming the potential is zero at infinity).